The present invention is more particr:larly concerned with a duct in which one or more optical fibre members are to be installed using a flowing fluid. Such a technique is described, for example, in EP-A-108590, and is referred to for convenience herein as the installation of optical fibre members by blowing. It is to be understood, however, that the flow of installation fluid, which is preferably air or some other suitable gas, could be produced by sucking, either in addition to, or instead of, blowing.
The ducts hitherto used for the installation of fibres by blowing have been made of plastics materials. It has been found that during the installation of an optical fibre member by blowing, a static electrical charge builds up on the inside wall of the duct, which then attracts the optical fibre member and impedes its progress. This is a variable phenomenon, depending on the particular combination of materials and on environmental conditions. Given time, the static charge will dissipate itself, but it is not normally practical to stop the installation process, perhaps several times, to allow this to happen.
Mobile additives have been incorporated in the duct material with a view to reducing friction between the optical fibre members and the duct. Ihese do in fact help to dissipate static electrical charge, though they were not proposed for this purpose. However, they are by no means ideal for this purpose. For one thing, the way these additives work is that they migrate to the surface of the duct, and this takes time. Furthermore, once there they are gradually removed by abrasion and other effects, such as washing, and so the effect of the additive in dissipating static electrical charge is gradually reduced. Also, the effectiveness of these additives can depend on the amount of atmospheric humidity.
Attention is also directed to WO90/00823 which proposes a duct having two concentric layers, the inner layer comprising, for example carbon doped high density polyethylene. Although proposed for the purpose of reducing friction between the optical fibre members and the duct the electrical conductivity of the carbon does in fact help to dissipate static electrical charge. This effect is not disclosed in WO90/00823.